Field of the Invention
The present invention relates to an imaging optical system having such an optical action that a light flux emitted from a certain point on an enlargement conjugate side is imaged at an intermediate imaging position, and is thereafter re-imaged on a reduction conjugate side, and to an image projection apparatus including the imaging optical system.
Description of the Related Art
In recent years, imaging optical systems to be used for an image pickup apparatus such as a digital camera and an image projection apparatus such as a projector have been required to have a compact entire system, a wide angle of field, and highly accurate optical performance over every object distance. An imaging optical system that utilizes the following intermediate imaging method (re-imaging method) is known to meet those requirements. Specifically, an enlargement conjugate point (object point) on an enlargement conjugate side (object side) is imaged on a reduction conjugate side (image side) in a manner that the enlargement conjugate point is once imaged at an intermediate imaging position within the optical system and is thereafter re-imaged at a reduction conjugate point.
In U.S. Pat. No. 5,748,385, there is disclosed an endoscope objective lens configured so that a primary image of a subject is formed by a front lens unit and then a secondary image of the subject is formed by a rear lens unit through a field lens. In U.S. Pat. No. 5,740,385, the rear lens unit is switched between two lens positions so as to vary an observation field of angle.
In US Patent Application Publication No. 2005/0080762, there is disclosed a superwide angle lens configured so that an image of a subject is formed on a primary imaging plane arranged in the vicinity of a field stop by an objective lens unit and the image formed on the primary imaging plane is re-imaged on a re-imaging plane (imaging element plane) by a relay lens unit. In US Patent Application Publication No 2005/0088762, the objective lens and a part or all of lens of the relay lens unit move to perform focusing. In addition, an image pickup element moves to perform focusing.
In Japanese Patent Application Laid-Open No. 2011-17984, there is disclosed an image pickup apparatus configured so that an afocal optical system is removably mounted on an object side of an imaging optical system. The afocal optical system includes, in order from the object side, an objective optical system, a field lens, and a correction lens unit.
A light flux from the object side is imaged in the vicinity of the field lens by the objective optical system, and is thereafter guided to the imaging optical system as collimated light by the correction lens unit. Then, the image formed in the vicinity of the field lens is re-imaged on an imaging plane by the imaging optical system. The correction lens unit and a part of lens units of the imaging optical system move to perform focus adjustment (focusing).